1. Field of the Invention
The present invention relates to a method for depositing a material having high single-crystallinity on a polycrystalline or amorphous material, and more particularly, it relates to a method of forming a single-crystalline oxide thin film on a material such as a metal tape. Throughout the specification, the term "single-crystalline" indicates a state dominated by crystals having specific orientations, in relation to not only a single crystal consisting of a specific orientation but a crystalline solid, formed by a mixture of crystals having different orientations, which is dominated by crystals having specific orientations.
2. Description of the Background Art
In order to provide a substrate consisting of a specific material with a single-crystalline thin film consisting of a different material, generally employed is a single-crystalline substrate having a lattice constant which is close to that of the material for the thin film to be formed. Then, the single-crystalline thin film is grown on the single-crystalline substrate by vacuum deposition, CVD, laser deposition or the like. This technique is well known as heteroepitaxial growth.
Also in the field of a technique for forming a thin film of an oxide superconductor, known is a technique of heteroepitaxially growing an oxide superconductor, particularly an yttrium-based ceramics superconductor on a single-crystalline substrate. In relation to this technique, laser deposition is particularly noted and energetically studied. For example, it is possible to epitaxially grow an yttrium-based oxide superconductor exhibiting excellent crystal orientation on an MgO single crystal by laser ablation.
In order to form a single-crystalline thin film having desired dimensions, shape and length, on the other hand, it is necessary to employ a polycrystalline substrate which is easy to obtain. This is because it is almost impossible to prepare a long tape-shaped single-crystalline substrate for forming a long cable run by a single-crystalline thin film. However, it is difficult to form a film strongly orienting a specific crystal orientation on a polycrystalline base material.
In recent years, there has been made an attempt for preparing a superconducting wire by forming an oxide superconductor on a flexible long tape base material. The base material for the wire is generally prepared from a metal, which is a polycrystalline substance in general. When an oxide thin film is formed on such a base material by laser deposition or reactive deposition, the thin film is generally in a polycrystalline state having random orientations or in an amorphous state. Even if the thin film has natural orientations, crystals forming the thin film orient specific crystal axes perpendicularly to the surface of the base material, while hardly orienting crystal axes in parallel with the surface. Also when an oxide superconductor film is formed on a polycrystalline substrate of MgO, SrTiO.sub.3 or ZrO.sub.2, orientations of crystal planes are irregular in the film as formed. Since a superconducting current is inhibited by grain boundaries, a thin film which is formed on a polycrystalline substance by a conventional technique cannot attain sufficient superconductivity.
In order to overcome such a disadvantage, there has recently been reported a method combining laser deposition and ion beam application (refer to Proceedings for 49th Teionkougaku-Choudendou Gakkai (Meeting on Cryogenics and Superconductivity), Spring 1993, p. 134). As shown in FIG. 5, according to this method, an intermediate layer of yttria stabilized zirconia (YSZ) is formed on a Hastelloy C276 tape 71 in a laser deposition device carrying out ion beam application. In this method, a YSZ target 72 is irradiated with a laser beam 73 for deposition, while the tape 71 is irradiated with a Kr.sup.+ ion beam 74 from a prescribed direction, to forcibly order crystals as grown. This literature reports that it has been possible to form a YSZ layer having crystal axes which are ordered in a plane parallel to the substrate by this method.
However, this method requires an ion beam gun, and hence the device as well as the process for the deposition are complicated. Further, it is difficult to stably drive the unit continuously for a long time by this method.